Perennials are plants that die down to their roots in winter and grow again the following spring. Some perennials grow so vigorously and spread so quickly that they become invasive and can crowd out other desirable plants. Not all of these perennials become invasive in all climates and locations. Some perennials that are considered invasive in warmer climates are held in check in colder northern areas. Check with your local County Extension Agent for information on perennials that spread quickly in your area, and whether or not they are considered invasive.
Beebalm (Monarda didyma) is a flowering perennial that grows 2 to 3 feet high. Its flowers are most often red, but other varieties are available with flowers in shades of pink. The flowers grow at the top of the stems in whorls and create an attractive, dense flower head. Although beebalm grows well in full sun, it has a tendency to spread more rapidly in shade. When cultivating beebalm, dig it up and divide the roots every three years. Replant one of the divisions and destroy the others. This will prevent them from rapidly spreading in your garden.
Lily of the Valley
One of the most popular ground covers for shady areas, lily of the valley (Convallaria magalis) spreads rapidly by underground runners. Just a few plants will quickly multiply and spread to fill in the area where it is planted. They will also spread into nearby lawn areas and have been known to come up between the cracks of adjoining concrete or natural stone walkways. Their spread can be controlled by installing garden edging with at least 4- to 6-inches of it buried underground.
Mint (Mentha sp.) is one of the most rapidly spreading culinary herb. It spreads by underground runners, by rooting at the nodes of branches that bend over and come into contact with the soil, and by producing seeds. A common practice by home gardeners to control the spread of mint is to plant it in a bottomless pot that is sunk into the ground, with the top edge of the pot at least 2-inches above the surface of the soil. To be effective, the portion of the pot that is buried should be at least 12-inches deep. It is also often grown outdoors in unburied pots that are set on stone or concrete to prevent the roots of mint from growing out of the holes in the bottom of the pot and into the ground. Mint is very hardy and will survive even severe winters outdoors planted this way.